When thrifting for wool sweaters there are clues I watch for:
• fiber content -- which for me is 100% wool
• labels that indicate it is hand knit or hand loomed
• a close look at the seams: a serged seam will indicate the possibility the item will contain only short pieces of yarn but a seam that has been chain stitched with matching yarn will indicate that the piece was knit back and forth with no break in the yarn. If there are no seams, even better, because the piece was knit in the round. The sweater pictured below has serged seams in the arms but the body was knit in the round.
Oxyclean and warm water in a 5 gallon bucket, stir well to dissolve the granules completely and then stuff the sweater into the water. Using a stick, I poke the sweater under the water and gently agitate it until it is completely wet. Then it sits overnight. I sometimes have to do this 2-3 times per sweater until the water rinses clear or dye is starting to release, then I know the wool is clean. Dry the sweater flat.
One of the sweaters from this batch was made from a hand spun yarn in New Zealand which resulted in 450gr of a DK weight yarn. Now to find a proper pattern to knit up.